1. Field of the Invention
The present invention relates to a charge/discharge protection circuit provided in a battery pack that supplies power to mobile electronic equipment, the charge/discharge protection circuit protecting a rechargeable battery of the battery pack from being damaged when the battery is placed in an overcharge condition, an overdischarge condition or an overcurrent condition. Further, the present invention relates to a battery pack in which the battery and the charge/discharge protection circuit are provided.
2. Description of the Related Art
In recent years, the proliferation of mobile electronic equipment in the art is remarkable. In the mobile electronic equipment, a battery pack containing a secondary or rechargeable battery is commonly used as a power supply that supplies power to the mobile electronic equipment. When the voltage supplied to the mobile electronic equipment by the battery pack is excessively decreased after the use over an extended period of time, the battery in the battery pack is recharged with a charger, and the battery pack can be repeatedly used again as the power supply for the mobile electronic equipment.
As disclosed in Japanese Laid-Open Patent Application No.11-103528, a charge/discharge protection circuit for protecting a battery of a battery pack against damage is known. In the charge/discharge protection circuit disclosed in the above document, the circuit elements to which only the battery voltage is supplied are constructed in a low-voltage resistant structure, and the circuit elements (or part of the integrated circuit semiconductor device) to which the charger voltage is supplied are constructed in a high-voltage resistant structure.
In the above charge/discharge protection circuit, the high-voltage resistant structure is needed only for the specific circuit elements. The above charge/discharge protection circuit can avoid the damage of the battery when it is placed in an undesired condition, and it is useful to downsize the integrated circuit chip for mounting the charge/discharge protection circuit.
However, in the above charge/discharge protection circuit, some circuit elements for detection of undesired conditions of the battery are connected at their inputs directly to the terminal to which the charger voltage is supplied.
When an improper charger, which supplies an excessively large voltage, is connected to the battery pack, the large voltage from the improper charger will be supplied to the above charge/discharge protection circuit. The above charge/discharge protection circuit detects that an overcharge condition of the battery occurs, and cuts off the connection between the improper charger and the battery. At this time, the potential of the terminal connected to the minus electrode of the improper charger is lower than the potential of the terminal connected to the minus electrode of the battery. The voltage at the inputs of the circuit elements of concern in the above charge/discharge protection circuit is lower than the potential of the terminal connected to the minus electrode of the battery. If this condition is kept on for a long time, the threshold voltage of the circuit elements used for the detection of the undesired conditions of the battery will be significantly varied. As a result, the characteristics of the circuit elements used for the detection of the undesired conditions of the battery will be significantly varied.
Since the above charge/discharge protection circuit is configured such that some circuit elements are constructed in the high-voltage resistant structure and the others are constructed in the low-voltage resistant structure, the variation of the threshold voltage of the circuit elements is more considerable than in a charge/discharge protection circuit configured with all the circuit elements built in the same structure.